As a conventional cooling method performed for an alignment device in an exposure apparatus, a temperature-managed fluorine-based inert coolant is generally circulated through a heating member such as a driving unit and a temperature adjustment device, as shown in FIG. 8. In the prior art shown in FIG. 8, the alignment device in the exposure apparatus is illustrated. A measurement distance 13 to an object 10 to be aligned is detected by using a measurement mirror 11 and a position measurement means 12, such as a laser interferometer, thereby measuring the position of the object 10 at high precision. A linear motor 1 is controlled by a controller 14 and a driver 15 on the basis of the measurement result. The linear motor 1 made up of stator 1a and rotor 1b is kept at a predetermined temperature by circulating a fluorine-based inert coolant 2 through a temperature adjustment device 6 and channel 7 in a temperature adjustment room. As disclosed in Japanese Patent Laid-Open No. 10-309071, the linear motor 1 has a jacket structure in which a coolant directly recovers the heat generated by a coil. As the coolant, the fluorine-based inert coolant 2 is used because of the following reasons.
1. The fluorine-based inert coolant is a chemically stable liquid, does not degrade or decay, and does not require any maintenance.
2. The fluorine-based inert coolant does not induce any rust and form any rust in a pipe or at a joint. Even if this coolant leaks, it hardly influences the interior of the apparatus.
3. The electrical insulating property of the fluorine-based inert coolant is very high (about 1015 Ω·cm). Directly cooling a coil or the like does not impair the insulating property.
A circulation cooling temperature for a coolant other than the fluorine-based inert coolant adopts a gas coolant such as air or carbonic acid gas, an antifreeze coolant such as oil or brine (ethylene glycol-based or propylene glycol-based), or water containing various additives such as a rust preventative and preservative.
The fluorine-based inert coolant has advantages described in the prior art, but also has the following disadvantages.
1. The unit cost is very high.
2. The warming potential is high.
3. The heat capacity (specific heat x density) per unit volume is as small as about one-half that of water.
The unit cost of the fluorine-based inert coolant is about 10 to 50 times higher than those of additive-containing water or various coolants such as brine. This increases the cost of an exposure apparatus which requires a large amount of coolant. The fluorine-based inert coolant does not decompose even in air owing to high chemical stability, and is pointed out to have a very high GWP (Global Warming Potential). The use of the inert coolant in an open system is, therefore, being reviewed, and for the user of the inert coolant in a closed circulation system, an alternate coolant is being examined for a long term.
In addition to this, a higher-output driving unit and higher cooling ability are demanded especially for the exposure apparatus. To improve the cooling ability, it is possible (1) to increase the coolant flow rate, (2) to decrease the coolant temperature, or (3) to increase the heat capacity of the coolant. As the coolant flow rate increases, necessary pump ability increases with its square. The pump becomes bulky and a higher flow rate is difficult to ensure. If the flow rate of a circulating coolant near an object to be aligned as an object subjected to temperature control is set to be higher than the conventional value, the coolant forms turbulence, vibrating a pipe or the like. The vibrations function as alignment disturbance, decreasing the alignment precision and further, the exposure precision. For this reason, the flow rate of the coolant cannot be simply increased. At an excessively low coolant temperature, air around the coolant path becomes nonuniform in temperature in comparison with the entire atmosphere. An interferometer laser for position measurement fluctuates in output, and the measurement precision and exposure precision decrease. From this, the use of an alternate coolant with a large heat capacity in place of the fluorine-based inert coolant has been examined.
Examples of such large-heat-capacity coolant are water (pure water) containing a rust preventive or preservative, and brine (coolant prepared by diluting an ethylene glycol-based or propylene glycol-based antifreeze with water). These coolants are actually used in various machine tools. If, however, water or brine is circulated as a coolant for a long time, rust forms on a metal surface of a pipe or the like that contacts the coolant, or the coolant decays due to breeding of unwanted bacteria or the like. To prevent this, water or brine containing a rust preventive or preservative is generally used as a coolant. Most rust preventives, however, contain a metal salt such as sodium ions or amine-based ions in order to dissolve the rust preventives in water. Many preservatives contain an amine-based component in addition to an alcoholic component in order to enhance the sterilization effect.
In addition, these coolants do not have the electrical insulating property of the conventional coolant, i.e., fluorine-based inert coolant, and the conventional structure of directly cooling an electrical component cannot be employed. Hence, a coolant which can ensure an electrical insulating property is required for the exposure apparatus instead of the fluorine-based inert coolant.
A semiconductor manufacturing factory must maintain a very clean space. Contamination of the atmosphere not only by a fine organic matter such as dust, but also by metal ions or amine-based organic ions must be minimized in terms of the semiconductor manufacturing process. Considering this, a coolant or the like used in the exposure apparatus preferably contains no metal salt or amine-based ions which act as contaminants (contamination) in case the coolant leaks from a pipe or the like. If these contaminants are at a negligible level for the factory, but the coolant leaking from a pipe, or the like, attaches to a precision surface plate, or the like, the coolant volatilizes to leave the additive component on the surface of the precision surface plate. The additive component may then influence the surface precision of the surface plate. In many cases, the precision surface plate serves as an alignment reference in the alignment device of the exposure apparatus. The decrease in precision seriously influences the alignment precision and exposure precision. Demands have therefore arisen for a coolant which does not leave any residue even upon volatilization.
In order to obtain a coolant having a large heat capacity and electrical insulating property, it has been examined to adjust the temperature in the exposure apparatus via pure water managed to 1 MΩ·cm or more (0.1 μS/cm or less). This also means that pure water does not contain any contaminant which adversely affects the manufacturing process of the semiconductor factory.
Implementation of such a process and coolant requires a pure water device and accessory device (deoxidation device or sterilization device such as a UV filter), resulting in a large equipment space.
Addition of a rust preventive or preservative also poses a maintenance problem. To maintain the effects of these additives, the concentrations of the additives must be managed. Since the concentration cannot always be monitored, it must be periodically checked at least every one or two months. This increases the maintenance burden of the semiconductor manufacturing apparatus, and further increases the burden on the user. To avoid the increase in burden, it is preferable to always monitor maintenance and management of the rust prevention effect.
In general, quality management of the coolant requires periodic maintenance. Most exposure apparatuses operate for twenty-four hours, and the maintenance burden is desirably decreased as much as possible.